[TheForge] Re: request for opinions (updated models)

[Mike Spencer]

> i would appreciate opinions on whether the bolts look physically
> correct.

That's ambiguous because "1/4-20 bolt" doesn't specify a bolt type.
The image is presumably "correct" for whatever type of bolt it's an
image of. :-)

F'rgzample, these 2nd-take images show an annular land under the head
such as is found on engine head bolts and the take-1 images did not.

lughaid> ...I think, though, that a small radius is needed where the
lughaid> shaft meets the head of the bolt...

Yes.  OTOH, bolts from some old machinery were cut singly by hand on a
lathe from hex barz, were heavily over-built for their purpose and
looked like this.  E.g. (because I've recently looked at them) the
bolts that attach the igniter to a 10HP Acadia one-lunger.

lughaid> Normally there is a short tapered area between the shaft and
lughaid> the start of the threads...

Yes.  But same as above for old, lathe-cut bolts.

lughaid>  ...then the crest of the thread should be the same diameter
lughaid> as the shaft of the bolt.

Well, or just a tad under.  You can typically put the threads of a
1/4" bolt through a 1/4" hole but not the unthreaded shank.  Here's
where you'd ask the CAD program for numerical dimensions or give it
numerical dimensions from the UNC thread spec.

lughaid> I also think the end of the bolts look a bit off. On most
lughaid> bolts the end of the shaft is deformed by the force of
lughaid> rolling or cutting the threads, so it looks a little dished
lughaid> out.

If this is a deformation incidental to the manufacturing process,
would you expect to include it as a spec within the CAD data?  Only, I
would think, if the presence (or absence) of the incidental feature
was relevant to the (engineering, artistic, whatever) design.

There's a nice little book called "The Nature of Art and Workmanship"
(David Pye, Studio Vista, London, 1971) that (among many other things)
contrasts a photo of a pop-top beer can with a hand-forged axe head.
The obvious chance ("stochastic" if you like technical words) elements
in the axe head wouldn't typically be part of the CAD spec whereas
nearly every easily visible element of the beer can, including finish,
would be specified. In fact, I got gouged once on this issue. [1]

If the under-head radius, taper above thread and/or pucker on the tip
are design specs, then presumably you want to be able to put them (and
put them :-) into the CAD data/image.  If not, not.

No?

---

[1] I had a commission to reproduce a jail door for the Halifax
    Citadel.  I was given a blueprint created by having a pencil guy
    draw the old barred door as accurately as possible and then add
    measurements.  They specified "hammered texture all over" and I
    did that with a great deal of trouble.  After delivery but before
    payment they brought it back and wanted the "hammered texture"
    removed.  I could have refused, having made what they asked for.
    BUT, the blueprint wasn't really a specification blueprint
    although it looked like one.  It was a dimensioned drawing of an
    old, hand made door with lots of small but measurable (and thus
    recorded) variations from uniform line and shape.  Of course, *my*
    door had lots of such variations, too, but not exactly the same
    ones.  E.g my 1" sq. was actually 1-1/16 in different places than
    their old one was.  Perfectly okay except that had I refused to
    slog through grinding the hammer marks off, they could have said,
    "Oh well, it doesn't match the blueprint anyway so we'll just
    forget the whole thing."

    I never did find out how their Mr. Whoever who pronounced on
    period authenticity could be happy with grinder marks but not
    hammer marks.


- Mike

-- 
Michael Spencer                  Nova Scotia, Canada       .~. 
                                                           /V\ 
mspencer at tallships.ca                                     /( )\
http://home.tallships.ca/mspencer/                        ^^-^^

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